It's a lot deeper than an apparent lack of conviction. Mentioning the possibility of a pivot completely changes the dynamic of the investment. 'No pivot' is asking "please invest in this startup; here's why it's a good idea". 'Pivot possible' is asking "please invest in me; I will find a way to get you a return on investment regardless of the means". Asking someone to invest in you as a person requires a certain level of egoistic thinking. Is the ego warranted? Maybe, but the investor won't know unless they build an extremely close rapport with you. Something like YC does with a 3-month on-premise bootcamp is that exact opportunity for investors to build a close rapport, so investing in people makes sense for YC. But a lot of investors are investing without building that rapport, so investing purely on the merits of business ideas makes more sense for them.